As more sophisticated uses of polymeric systems are devised, researchers have attempted to meet the demand for development of new polymeric systems which exhibit properties tailored to particular end uses. New and useful polymeric systems have been developed which contain various additives designed to enhance particular properties of various polymers without imparting a substantial adverse effect on the inherent properties of such polymers. Enhanced properties have also been achieved by blending various polymers to achieve a balance of properties which are exhibited differentially by the individual polymers. Advantages gained by such blends typically relate to processing, for example, polymer rheology, or properties such as adhesion and toughness.
Unfortunately, the mixing of numerous combinations of individual polymers which possess highly desirable properties generally results in a phase separated mixture wherein the mixture of individual polymers does not provide a thermodynamically miscible blend. Decreased miscibility of the individual polymers comprising the blend typically results in a system which offers properties inferior to those of the individual polymers making up the blend. Thermodynamically miscible blends are assured of mechanical compatibility. While phase separated blends can offer useful properties, they often require modification in order to provide compatibility and to yield a system having a property balance of commercial significance.
The poor performance of phase separated blends of various polymers is typically caused by structural incompatibility (due to poor interfacial adhesion) wherein the respective polymers tend to interfere with each other's performance causing an unacceptable degradation in one or more of the desired properties. For example, admixture of poly(vinyl acetate) and homopolymers of styrene or acrylic acid results in an immiscible blend exhibiting poor mechanical compatibility.
Likewise, direct copolymerization of poly(vinyl acetate) and acrylic acid is not commercially practical because the reactivity of acrylic acid toward polymerization is significantly faster than that of vinyl acetate such that the initial polymer formed in a typical free radical polymerization is very high in acrylic acid content. After the acrylic acid is depleted, poly(vinyl acetate) is produced. The polymers of acrylic acid produced during the early stages of the polymerization are not miscible with the poly(vinyl acetate) produced during the later stages of the polymerization. The resulting phase separated blend typically exhibits inferior mechanical properties and will appear to be inhomogeneous. While the random copolymerization of poly(vinyl acetate) and poly(acrylates) can be achieved to some degree of success by the controlled addition of the more reactive monomer, the procedure is very difficult to control and reproduce particularly at higher (&gt;5%) acrylic acid contents.
The immiscibility of important polymers has been noted in the literature. Hsieh and Wong (J. Chin. I. Ch. E., 19(10) 17(1988)) note the phase separation of blends of poly(vinyl acetate) and polystyrene. The investigators prepared thermodynamically miscible blends by formulating copolymers based on vinyl acetate/acrylic acid copolymers and styrene/4-vinyl pyridine copolymers. The paper fails to disclose blends of styrene/acrylic acid copolymers with poly(vinyl acetate).
Considerable research is being conducted in order to identify thermodynamically miscible polymer blends which offer the mechanical advantages of the respective polymers making up the blend while obviating the problems incurred in attempting to copolymerize the monomers. Olabisi, et. al., Polymer-Polymer Miscibility, pages 238-9, Academic Press, NY (1979), presents a survey of miscible polymer systems which includes poly(vinyl acetate) and vinyl acetate copolymers comprising blends of poly(vinyl acetate) and poly(vinylidene fluoride) and blends of poly(vinyl acetate) and poly(vinyl nitrate).
Saunders, K. J., "Poly(vinyl acetate) and Related Polymers", Organic Polymer Chemistry, pages 104-115, Chapman and Hall, London (1973), describes several different routes for preparing vinyl acetate monomers and discusses emulsion polymerization techniques for producing poly(vinyl acetate). The properties, applications in films and solubilities of the resulting polymers are discussed. Copolymers of vinyl acetate and alkyl acrylates, fumarates and maleates are described as is the conversion of poly(vinyl acetate) to poly(vinyl alcohol) and poly(vinyl acetals).
A need exists in the art for the preparation of blends of poly(vinyl acetate) and acrylic acid-containing copolymers and terpolymers which exhibit thermodynamic miscibility wherein single phase behavior is observed and wherein the resulting blends would be stable over sufficient periods of time to allow for processing and use in desired applications. Such thermodynamically miscible blends would be particularly useful in adhesive and coating applications.